Carotenoids are a class of substances widely distributed in nature and having very interesting properties. Accordingly, there is great interest in industrially realizable syntheses. The Wittig reaction plays a very important role in the formation of the carotenoid polyene chain. It has been investigated extensively and also used industrially.
Carotenoids (intermediates and final products), e.g., carotenes and xanthophylls, are in the normally desired (all E) configuration poorly soluble in most solvents. Aromatic hydrocarbons and especially halogenated, lower aliphatic hydrocarbons, such as chloroform and methylene chloride, occupy as solvents a certain special position because of their good ability to dissolve carotenoids, and have hitherto preferably been used for this purpose. However, at present the dwindling acceptance of such solvents is disadvantageous for known and justified reasons. In addition, carotenoids have a typical property of incorporating solvents in non-stoichiometric amounts. Residues of the aforementioned solvents are subject to strict regulations, which, moreover, are not the same everywhere. The thermal stability of carotenoids is, however, frequently critical and therefore the removal of the incorporated traces of solvent is difficult and generally can be realized only at the expense of the commencement of decomposition.
The known industrial processes can be divided into the following sub-groups:
1) Homogeneous processes, in which the educts and the product remain in solution during the entire reaction; the isolation of the end product is carried out by precipitation, typically associated with an isomerization step. PA0 2) Two-phase processes, in which the educts are dissolved in an organic solvent to which the required base is added, but in aqueous solution. For the working up, the phases are separated from one another and the organic phase is worked up similarly as in 1). PA0 3) Heterogeneous processes, in which the educts are used in undissolved form. However, a clear solution results at least temporarily during the reaction. On the other hand, there are also reverse embodiments in which the reaction is started as a solution and the product precipitates during or after the actual reaction.
The Wittig reaction, which involves the reaction of a carbonyl compound such as an aldehyde or ketone with a triphenylphosphine compound known as a ylide, is generally carried out under basic conditions or at least in the presence of an acid-binding substance. As a waste product of the Wittig reaction there always results one equivalent of triphenylphosphine oxide, which has to be separated from the reaction mixture. The choice of solvent has a decisive influence on the working up.
The most important side reaction which generally occurs in the case of a Wittig reaction is the hydrolysis of the phosphonium salt: see the following Reaction Scheme.